(1) Field of the Invention
The present invention relates to a laser beam scanner for reading a bar code or the like by utilizing holograms and laser beams. In particular, it relates to an improved hologram scanner in which the height thereof is reduced.
A hologram scanner comprises holograms assembled in an optical scanning system and includes a laser beam for scanning a predetermined scanning pattern. Such a hologram scanner makes it possible to employ a complicated scanning pattern with a simple optical system and realizes a bar code reader having a high reliability and accuracy.
Hologram scanners are utilized in supermarkets or the like as terminal equipment of a POS (point of sales) system which manages sales data in Real Time by a direct input of the sales data to a computer from each cash register.
At a supermarket, each scanner is placed on a register table with the cash register and a bar code marked on a sales article is read by the scanner while the article is moved thereabove by hand. In such a situation, the bar code reader must be small in size, especially in height, from the view point of the space needed for each scanner, the layout of the scanners, and the operation of the scanner.
Reducing the height of the bar code reader is urgently required at, for example, supermarkets in Europe, where the sales clerk is obliged to sit to operate the bar code reader.
As mentioned above, the hologram scanner comprising holograms and assembled in the optical scanning system is widely utilized as a laser beam scanner which scans a predetermined scanning pattern by a laser beam. The hologram scanner utilizes holograms to simultaneously function as a conventional rotatable polygon mirror and a conventional scanning lens system, thus making it possible to arrange a complicated scanning pattern with a simple optical structure, and to realize a bar code reader having a high reliability and accuracy.
However, there is a need for a further simplification of the structure of the scanner, to reduce the height thereof when the scanner is used as a bar code reader at a supermarket or the like, wherein the scanner is placed on a counter table with a cash register and a bar code marked on a sales article is read by the reader while the article is moved thereabove.
(2) Description of the Related Art
FIG. 10 shows a conventional laser beam scanner according to the prior art and FIG. 11 is a perspective view of a stationary hologram of a prior art.
In FIG. 10, the prior art scanner comprises a laser beam scanning system 10 comprising a motor 11 and a hologram disc 12 having a plurality of hologram facets, a mirror 13, a stationary hologram 2, a window cover 14, and a scanning window 15 (which corresponds to an uncovered portion of the stationary hologram 2 through an opening 14a of the window cover 14). A scanning beam 16 diffracted through the hologram disc 12 is reflected by the mirror 13 and reaches the stationary hologram 2, where the scanning beam 16 is further diffracted and impinged on a bar code 17 marked on an article 1 to be scanned.
Scattered light 18 reflected from the bar code 17 propagates in the direction opposite to the scanning beam 16, as illustrated by dash lines, and reaches the hologram disc 12, where the signal light 18 is diffracted toward an optical detector (not shown) which reads the bar code 17.
The scattered signal light 18 reflected from the bar code is divergent until reaching the stationary hologram 2. However, the reflected light is not overly diverged or widened since the stationary hologram 2 is located in the vicinity of the bar code 17. The scattered divergent light 18 is converted to a parallel plain wave light by the stationary hologram 2 and propagated to the hologram disc 12 without divergence, thus reducing the light receiving area of the hologram disc 12.
The scanning beam 16 scans the surface of the bar code 17 along a scanning line. In order to reliably scan the entire bar code 17 irrespective of the location and movement of the article 1, a plurality of scanning lines are arranged in different directions.
As illustrated in FIG. 11, the stationary hologram 2 comprises a transparent substrate 21 constituting a scanning window 15 and hologram strips 22, 23, and 24 formed on the scanning window 15. A scanning pattern comprising a plurality of scanning lines 220, 230, and 240 corresponding to the hologram strips 22, 23, and 24 and intersecting each other is formed above and apart from the stationary hologram 2.
The problems of the above-mentioned conventional laser beam scanner are explained with reference to FIGS. 12a and 12b and FIGS. 10 and 11.
With the arrangement of FIG. 10, wherein the window cover 14 and the stationary hologram 2 having the hologram strip arrangement of FIG. 11 are arranged in close proximity, the bar code 17 is not always scanned by the scanning beam, depending on the location and direction of movement of the bar code 17 above the scanning window 15. In FIG. 12a for example, the bar code 17 being moved along a dash line .circle.1 is read but the bar code being moved along a dash line .circle.2 or .circle.3 is not read. This is because, if the bar code 17 is angled as depicted in FIG. 12(a), only the scanning line 240 can pass through the entire length of the bar code from end to end, whereas the scanning lines 220, 230 cannot pass through the entire length of the bar code due to the difference of the inclination angle of the scanning line with respect to the bar code 17. Therefore, the bar code 17 being moved along a dash line .circle.2 or .circle.3 cannot be read since the scanning line 240 does not intersect the dash line .circle.2 or .circle.3 in a close vicinity above the scanning window.
On the other hand, the scanning lines 220, 230, 240 intersect each other at a level sufficiently away from the scanning window as illustrated in FIG. 12(b). Therefore, the bar code 17 can be read by the scanning line 240 irrespective of the moving line of the bar code 17 (dash line .circle.1 , .circle.2 , or .circle.3 ). Therefore, the scanning window must be arranged at the level where the scanning lines intersect each other. The height of the level from the window surface of the stationary hologram 2 is more than 100 mm in the conventional hologram scanner, thus making the height thereof considerably high.